1. Field of the Invention
The present invention relates to a process for recycling a catalyst used for preparing polyoxyalkylene polyol and a preparation process of the polyoxyalkylene polyol. More particularly, the invention relates to a process for recycling the polymerization catalyst of alkylene oxide in the preparation of a polyoxyalkylene polyol which is used for a raw material of polyurethane resin and a surface active agent, and to a preparation process of the polyoxyalkylene polyol by ring-opening addition polymerization of alkylene oxide in the presence of the recycled catalyst.
2. Description of the Related Art
Polyoxyalkylene polyol which is used for a polyurethane raw material is usually prepared by conducting ring-opening addition polymerization of alkylene oxide on an active hydrogen compound initiator in the presence of an alkaline catalyst. In the preparation of polyoxyalkylene polyols, various suggestions have been conventionally made concerning the improvement of their productivity and product qualities.
It has been known in the preparation of polyoxyalkylene polyol that an alkylene oxide increases it's rate of addition polymerization when the alkylene oxide has a high concentration in the reaction, when the reaction temperature is high, and when the amount of the polymerization catalyst, i.e., an alkali metal compound is increased. Improvement of productivity by such methods increases at the same time the content of an unsaturated bond due to a side reaction.
The unsaturated bond results from monool which is formed by the side reaction of propylene oxide, a kind of alkylene oxide, and has a terminal double bond. The monool gives an adverse effect on the quality of prepared polyoxyalkylene polyol and leads to unfavorable results such as reduction in the physical properties of polyurethane resins.
Consequently, the effect of polymerization conditions on the increase in the monool formation has been studied. It has been reported in Kobunshi Ronbunshu, 50, No. 2, 121-126, (1993) that high temperature reaction gives the largest effect on the increase in the unsaturated bond, and that the concentration of alkylene oxide and the concentration of potassium hydroxide polymerization catalyst has a relatively small influence on the formation of unsaturated bond. However, a limitation is imposed upon the pressure resistance of a reaction vessel and the concentration of alkylene oxide is relatively difficult to increase.
It can be carried out with extreme ease to raise the concentration of potassium hydroxide polymerization catalyst. The polymerization catalyst, however, is separated after reaction from crude polyoxyalkylene polyol by neutralization with an acid or washing with water and is thereafter abandoned. Consequently, an increase in the amount of the potassium hydroxide polymerization catalyst gives an adverse effect on the material balance due to adhesion of the product to the salt which is formed by neutralization of the polymerization catalyst or due to the increase in the amount of the washing water and leads to elevation of product cost.
U.S. Pat. No. 3,393,243 has proposed a process for reducing the amount of unsaturated bonds in polyoxyalkylene polyols by using an alkali metal compound other than potassium hydroxide as a polymerization catalyst. Monool formation is suppressed in the case and the process is characterized in that even productivity improvement by faster reaction rate gives no adverse effect on the quality of the product.
In the above reference, a cesium compound is used as the alkali metal compound polymerization catalyst in order to reduce unsaturated bonds in the polyoxyalkylene polyol obtained. After finishing polymerization, the reaction mixture is neutralized with a neutralization agent composed of water and oxalic acid, dehydrated and filtered to remove the cesium salt. However, no description is found on the recycling of the cesium compound catalyst. The cesium compound is very expensive and the process has a defect of being less economical.
Various techniques for recycling the cesium compound catalyst have been investigated. For example, crude polyoxyalkylene polyol by polymerization in the presence of a cesium compound catalyst is neutralized by adding an organic acid such as oxalic acid and water. The cesium salt thus obtained is separated. A mixture of the separated cesium salt and the polyoxyalkylene polyol adhered thereto is calcined to form cesium oxide, cesium oxide is successively hydrated to recover cesium hydroxide, and the recovered cesium hydroxide is reused as a polymerization catalyst for preparing a polyoxyalkylene polyol. The process, however, has a problem of cost increase resulting from burning off the organic acid for use in neutralization and a problem of fuel cost required for the calcination, and is thus unfavorable for cost reduction.
In another technique, the crude polyoxyalkylene polyol as intact which is obtained by polymerization in the presence of a cesium compound catalyst, or an aqueous solution of a cesium salt obtained by neutralizing the crude polyoxyalkylene polyol with an organic acid and/or a mineral acid and water, is brought into contact with a cation exchange resin to adsorb a cesium ion to the cation exchange resin. Successively the cesium ion is recovered by a basic eluent in the form of cesium hydroxide which is a polymerization catalyst in the preparation of polyoxyalkylene polyol. The process, however, has a problem of reducing the activity and the efficiency as a polymerization catalyst of alkylene oxide because the eluent contaminates the aqueous solution of cesium hydroxide and decreases purity of cesium hydroxide.